The radio controlled timepiece is a timepiece provided with a function of receiving a standard wave (40 kHz radio wave, for example) including time information and date information by a cesium atomic clock with an accuracy of 1 second in 1 million years by a built-in antenna for correcting a time error. Thus, accurate time can be displayed all the time as compared with a quartz timepiece in which an error of approximately 20 seconds per month is generated, and labor to correct time can be omitted and thus, the radio controlled timepiece has been rapidly prevailing in these days.
In the radio controlled timepiece, its receiving performance is determined by an antenna characteristic and a receiving-circuit characteristic.
As a type of the antenna, a coil-like bar antenna in which a conductor is wound around a core formed by a ferromagnetic body and the like is generally used since the wavelength of the standard wave is as long as approximately 5 km and resonance by a wavelength direction in the wrist watch is difficult. An electromotive force is obtained at a coil by a magnetic flux transmitted through the bar antenna.
A receiving circuit detects time information based on an output of the antenna and transmits it to a watch movement. The watch movement refers to complex components in which a battery and circuit elements such as a count circuit required for a watch operation are collected up in a single unit. Depending on the cases, a time display measure such as a dial plate, a liquid crystal display device and the like may be included.
The wrist-watch type radio controlled timepiece has a role as an article of taste other than a function as a watch, and its design and texture are important. Particularly, a sense of high-class feeling as a watch is an important element, and metal exterior constituting the exterior of a watch body by metal is desired.
However, an output of an antenna is extremely lowered if housed in the metal exterior. That is because an eddy current is generated on the surface of the metal exterior, which makes difficult for the standard wave to enter the inside of the watch and a magnetic flux generated by an electric current flowing through the antenna is prevented by a counter magnetic flux by the eddy current generated on the surface of the metal exterior and the current flowing through the antenna is prevented.
That is, the wrist-watch type radio controlled timepiece has a problem that the standard wave is difficult to be received with the metal exterior though it is desired. Thus, there are many proposals made for an art with which receiving is possible even in the metal exterior. For example, it is an art to improve the shape of the antenna. By increasing a receiving sensitivity of the antenna, more standard wave can be received.
The wrist-watch type radio controlled timepiece uses a bar antenna in many cases as mentioned above, and an art that the receiving sensitivity is increased by improving the shape of a magnetic core of the bar antenna is known (See Patent Document 1, Patent Document 2, for example).
FIG. 26 is a diagram for explaining a related art shown in Patent Document 1 and is modified for ease of explanation without departing from its gist. In FIG. 26, reference numeral 100 denotes a watch exterior, 102 for a magnetic core, 103 for a coil wound in the longitudinal direction of the magnetic core 102, and 104 for an extension portion of the magnetic core 102. The coil 103 actually has a thin wire wound in order, but for ease of understanding, it is shown cylindrically. In addition, portions not necessary for the explanation are omitted.
As shown in FIG. 26, the related art shown in Patent Document 1 is provided with the extension portion 104 on both sides of the magnetic core 102, and by directing the extension portion 104 toward the side of a windshield glass (not shown), much more magnetic flux is led to the magnetic core 102 with respect to the standard wave incident from the windshield glass side.
FIG. 27 is a diagram for explaining a related art shown in Patent Document 2 and is modified for ease of explanation without departing from its gist. In FIG. 27, reference numeral 100 denotes a watch exterior, 102 for a magnetic core, 103 for a coil wound in the longitudinal direction of the magnetic core 102, 104 for an extension portion of the magnetic core 102, and 106 is a radio-wave collection plate connected to the extension portion 104. Similarly to FIG. 26, the coil 103 is shown cylindrically. Note that, portions not necessary for the explanation are omitted.
As shown in FIG. 27, in the related art shown in Patent Document 2, the radio-wave collection plate 106 is connected to the extension portion 104 of the magnetic core 102 so as to be curved in the watch exterior 100. With such configuration, more standard wave entering the watch exterior 100 can be led to the magnetic core 102 and in addition, directions where the radio controlled timepiece can receive the standard wave are increased. The wrist-watch type radio controlled timepiece is worn on an arm and its direction is changed in various ways according to an action of a human body, and increase in the directions capable of receiving a wave is effective.
Patent Document 1: Japanese Patent Laid-Open Publication No. 2005-3675 (page 15, FIG. 11)
Patent Document 2: Japanese Patent Laid-Open Publication No. 2004-235701 (pages 5 to 7, FIG. 3)
The related arts shown in Patent Document 1, Patent Document 2 both consider the case where the watch exterior 1 is metal and a flow of the magnetic flux is not prevented by the metal exterior. However, with these structures, it is known that more standard wave cannot be received.
That is, with the related art shown in Patent Document 1, since the extension portion 102 is merely extended from the magnetic core, when seen from the windshield glass side (not shown), of the watch exterior 1, the extension portion 102 is small in the planar view and is not in a structure to receive more standard wave. With such structure, the magnetic flux cannot be obtained effectively.
Moreover, if a time display measure such as a dial plate, a liquid crystal display device and the like is provided on a top portion of the antenna, they shield the standard wave, and even with presence of the extension portion 102, the standard wave can not be received effectively.
In the related art shown in Patent Document 2, a wave catching portion 115 is formed in a fan shape when seen from the windshield glass side (not shown), of the watch exterior 100, which is suitable for leading the standard wave incident from the windshield glass side to the magnetic core, but similarly to the related art shown in Patent Document 1, if the time display measure is provided at a top portion of the wave catching portion 115, it shields the standard wave and the wave catching portion 115 can not effectively receive the standard wave any more.
For the recent wrist-watch type radio controlled timepiece, more size reduction is in demand and the size reduction of the watch exterior has progressed. However, the time display measure is preferably bigger as long as time information is to be seen more easily. For example, if the time display measure is a hand-type analog-display dial plate, devices have been made to thicken the hands.
That is, the standard wave is getting more difficult to be received with size increase of the time display measure and the hands.
In addition, if the antenna end portion is too close to the metal exterior in the wrist watch constituted by the metal exterior, an eddy current loss is generated between the metal exterior and the antenna, which causes a situation of loss of antenna gain. Further, if the antenna is brought into contact with the metal exterior, the eddy current flows to the antenna and the entire metal exterior, and the antenna output is lowered. Thus, it is necessary that the antenna and the exterior should be separated by an appropriate distance.
Furthermore, since a loss is also generated inside of the antenna due to the eddy current and hysteresis loss, immoderate increase of the size of the antenna will enhance the influence of these resistance factors rather than the capability of collecting the magnetic flux, and the antenna gain will be lost to the contrary.
Besides, if self-inductance of the antenna is raised by size increase of the antenna, a self-resonant frequency of the antenna is lowered, and a frequency band that can be used as an antenna is narrowed.
As obvious from the explanation, with the related arts shown in Patent Document 1, Patent Document 2, the provision of the time display measure for notifying time, which is an essential purpose of a watch, more easily to be seen and the receiving of more standard wave can not coexist at the same time.
Therefore, a wrist-watch type radio controlled timepiece having solved such problems has been in a strong demand.
A technical object of the present invention is to solve such problems and to provide an antenna that can ensure a sufficient radio receiving level and a radio-wave receiving device equipped with the antenna. Particularly, the technical object of the present invention is to provide an antenna for radio controlled timepiece that can be housed in the wrist-watch type radio controlled timepiece for which small size and light weight are required, and the radio controlled timepiece provided with the antenna for radio controlled timepiece and an easy-to-see time display measure, which can ensure sufficient receiving level of the standard wave.